1. Field of the Invention
The present invention relates to an apparatus for generating ions suitable as a source of corona ions for forming electrostatic latent images in an electrostatic printer.
2. Description of the Background Art
In a conventional apparatus for generating ions to be used as a source of corona ions for forming electrostatic latent images in an electrostatic printer, there is one such apparatus comprises a corona charger or a solidified ion generation substrate, and an ion current control electrode on which a multiplicity of slits corresponding to recording dots are provided. In this apparatus, a flow of ion currents towards a recording medium is allowed or disallowed by controlling a high voltage applied to the ion current control electrode. In particular, with the solidified ion generating substrate, it has been possible to generate highly density corona ions suitable for high speed recording.
This type of an apparatus for generating ions is disclosed in U.S. Pat. Ser. No. 4,155,093, which is schematically shown in FIG. 1.
As shown in FIG. 1, two electrodes 102 and 103 are provided above and below an insulative substrate 101, of which the electrode 103 has an incision or hole 104 for increasing a field concentration so as to generate corona ions more easily. Between these electrodes 102 and 103, an alternating voltage 105 is applied, so that a strong alternating field is created in the incision or hole 104 by which a high density of positive and negative ions are generated. Of these generated ions, only the negatively charged ones are selectively allowed to flow towards acceleration electrodes 107 as ion currents by means of a control voltage 106 to be applied to the electrode 103. These ion currents are accelerated by a voltage 108 applied to the acceleration electrodes 107 so as to reach an insulative recording medium 109 on which an electrostatic latent image is to be formed.
A so called ion recording head is a collection of as many apparatuses for generating ions of the type described above as a number of picture elements required. Such an ion recording head is known to have the following drawbacks. First, because both of the positive and negative corona ions are steadily generated by the electrodes for generating ions, a lifetime of the recording medium is shortened and at the same time an ozone odor is produced as the corona ions leak out. Second, the control voltage to be applied to the control electrode is required to be a high voltage of over 150 V. As a consequence, since a control IC for controlling such a high voltage inevitably occupies a large mounting area which is prohibitive for a highly condensed implementation, the realization of a highly compact ion recording head has been difficult.
As a method of reducing the control voltage, a proposition was made in Japanese Patent Application Laid open No. S61-255870 a control voltage was applied in a direction perpendicular to the slits, for the corona ions to pass through. According to this proposition, it is possible to reduce the control voltage to a low of approximately 30 V. However, since it is necessary to provide additional electrodes for producing a field perpendicular to the slits, a structure is further complicated. This gives rise to a limitation in terms of a mounting area, which in turn gives rise to limitations on the resolution of the image and the number of picture elements that can be incorporated.
In addition to these problems of conventional apparatuses for generating ions, there is a general problem associated with any apparatus for generating ions. Namely, ion generation is affected by the environmental conditions of the apparatus a critical voltage for the corona ion generation and the amount of corona ion currents changes, and the corona ion generation becomes uneven, as the environmental condition of the apparatus changes. Among the environmental conditions that affect corona ion generation, the temperature affects the critical voltage for the corona ion generation, whereas the atmospheric pressure affects the amount of the corona ion currents and the critical voltage. Also, vapor condensation on the electrode for generating ions occurring at high humidity can prevent corona ion generation altogether.
More specifically, the effects of environmental conditions on the corona ion generation by an apparatus for generating ions can be analyzed as follows.
When a pair of parallel electrodes in the apparatus which are provided on an insulator are approximated by a pair of parallel wires, the critical voltage for corona ion generation is given by: ##EQU1## where 2a is equal to a thickness of the electrodes (cm), L is a distance between the electrodes (cm), P is an atmospheric pressure (cmHg), T is a temperature (0), m is a coefficient depending on the cleanness of the surface of the electrodes, which is equal to 1 when the surface is clean (See R. M. Shaffert "Electrophotography", p.235, Focal Press, London, 1980). According to these equations (1) and (2), for the apparatus where L=100 micron and a=10 micron, the critical voltage V.sub.T is roughly 650 V at 250 and 76 cmHg.
The dependence of the critical voltage on temperature is shown in FIG. 2. As can be seen from FIG. 2, the critical voltage at 0.degree. is roughly 60 V higher than that at 25.degree.. In fact, for a given ion current, the control voltage must be roughly 60 V higher at 0.degree. than at 25.degree..
The dependence of the critical voltage on atmospheric pressure is shown in FIG. 3. As can be seen from FIG. 3, the critical voltage at 71 cmHg (950 mb) is roughly 40 V lower than that at 76 cmHg (1013 mb). In fact, for a given amount of ion currents, the control voltage needs to be roughly 40 V lower at 71 cmHg than at 76 cmHg. Furthermore, because the mobility of the corona ions is inversely proportional to the atmospheric pressure, the amount of ion currents changes slightly, and accordingly there is a slight shift of curves in FIG. 3 as indicated by a one dot chain line.
Thus, the critical voltage for corona ion generation is greatly affected by the temperature and the atmospheric pressure, while the amount of corona ion currents is also affected by the atmospheric pressure to a smaller extent. It is to be noted that these environmental conditions usually do not change very much during a particular operation of the apparatus, so that once the operation is started out successfully, a fairly stable operation can be expected.
On the other hand, when vapor condensation on the electrode for generating ions occurs from high humidity, corona ion generation is prevented altogether. In this condition, if the control voltage is increased to approximately 900 V insulation by the air is lost, and the spark discharge occurs, as shown in FIG. 4, which in turn causes the breakdown of the electrodes.
In the apparatus for generating ions using a solidified ion generation substrate, resistor heat elements for removing the vapor condensation on the electrodes may be provided. Alternatively, a high frequency voltage which is lower than the critical voltage may be applied between the electrodes before operation, so as to heat up the electrodes through the insulator by the induction loss of the insulator, as disclosed in Japanese Patent Application Laid Open No. 63-18372.
An apparatus described in the last reference is schematically shown in FIG. 5. In this apparatus, a high frequency voltage is applied between a discharge electrode 111 on an inductive body 110 and an induction electrode 112 embedded in the inductive body 110 by a voltage source 113 controlled by a voltage controller 114 in order to generate corona ions, and either the generated positive and negative corona ions are selected by a bias voltage 114 as corona ions to charge a recording medium 116. In addition, a heater 117 is provided on the inductive body 110 in order to maintain the electrodes 111 and 112 at a constant temperature by controlling a heater power source 18 in accordance with the temperature of the electrodes 111 and 112 detected by a temperature detector 119. By means of these features, the temperature of the electrodes 111 and 112 is controlled to be constant as shown in FIG. 6.
Meanwhile, as shown in FIG. 7, a high frequency voltage V.sub.A, which is less than the critical voltage V.sub.T, is applied for a predetermined period of time between the electrodes 111 and 112 so as to accelerate heating by the heater 117 by the heat generation by the inductive body 110 due to the induction loss of the insulator. Thus, by maintaining the temperature of the apparatus above that of the environment, the vapor condensation on the electrodes 111 and 112 is removed, and then a control voltage V.sub.0, which is greater than the critical voltage V.sub.T, by V.sub.C is applied. These high frequency voltage V.sub.A and the control voltage V.sub.0 are biased by the bias voltage V.sub.B.
However, in this apparatus of FIG. 5, temperature control is not performed in accordance with the humidity, depending on which the amount of the vapor varies considerably. Moreover, whether the vapor is completely removed from the electrodes 111 and 112 is not checked at all.
Also, in this apparatus of FIG. 5, no attention is paid to any change in atmospheric temperature or atmospheric pressure, so that the apparatus is still greatly affected by environmental conditions.
As for a corona charger in which a high voltage is applied to a wire in order to generate corona ions, which has been widely used in conventional copy machines, no attention has been paid to the effects of environmental conditions, so that fluctuations in the quality of the copied images has been a general feature in conventional copy machines.